Slobodan Milosevic is dead, but a new trial touching the heart of his regime has begun in The Hague.

The prosecution at the United Nations war crimes tribunal of Jovica Stanisic, the former chief of Mr. Milosevicâs secret police and a man once considered the second most powerful official in Serbia, started on Monday.

During Serbiaâs wars against Croatia and Bosnia in the 1990s, Mr. Stanisic was believed to be a key strategist of campaigns, often fought through covert police actions, that terrorized and killed many civilians.

Also on trial is Franko Simatovic, the chief of the secret policeâs special operations unit, which was in charge of training covert groups and directing them in the field. Prosecutors say the two men acted on direct orders from Mr. Milosevic, Serbiaâs president, who trusted his secret police more than his military during the wars.

The defendants were arrested in 2003. Each faces five counts of murder, persecution, forced deportations and inhuman acts during the Balkan wars. Both men have pleaded not guilty.

Prosecutors have said that in the spring of 1991, Mr. Stanisic created a large covert fighting force that was like a parallel army. They say that under the oversight of the Serbian Ministry of Interior, fighters were trained at more than two dozen camps, with many posing as volunteers. They fought in Bosnia and Croatia, prosecutors said, often acting as hit men who moved ahead of military columns to terrorize, expel and kill thousands of non-Serb civilians from lands that the Milosevic regime intended to be for Serbs only.

The operations of the secret police in Croatia and Bosnia have not been fully exposed in court and have been little debated in Serbia. The covert units were designed to give the Serbian government and its military deniability for some of the most brutal campaigns against civilians.

At the tribunal, several other trials involving high-level officials are under way or awaited. But lawyers and human rights groups are following the case against the two secret police chiefs with special interest because of their close ties to Mr. Milosevic. His trial ended inconclusively when he died in his cell in 2006.

There are many links between the Milosevic trial and the charges against his secret police officials, and judges have decided that some of the secret testimony and documents from the Milosevic proceedings, which had remained confidential, can now be used and disclosed at this trial.

Mr. Simatovic, 58, was in court on Tuesday as prosecutors concluded their long opening statement. But Mr. Stanisic, 57, has not appeared at any recent hearings, because he has been diagnosed with kidney problems and severe depression. After postponing the trial four times because of his illness and medical treatment, judges ordered a video link installed in prison for Mr. Stanisic. But prison officials said he had refused to watch the proceedings.

The presiding judge, Patrick Robinson, said the trial could start, despite a doctorâs warning that the case âcould harm the health of the accused.â He said Mr. Stanisicâs condition would be closely monitored.

Soon after the long-awaited trial began, however, new delays loomed. Mr. Stanisicâs lawyer, Geert-Jan Knoops, said that he wanted to resign from the case; he said that requiring his client to watch the proceedings via video from a cellblock was unique in international law.

Prosecutors said they would present evidence that a number of violent paramilitary groups like Arkanâs Tigers, the Scorpions, Frenkiâs men and others that rampaged through towns and villages, expelling and killing non-Serbs â and often looting cars, cattle and valuables â were not rogue bands of criminals or volunteers, but well-trained, well-equipped and well-paid fighters connected with the secret police.

âMilosevic was regularly informed of all activities through Stanisic and Simatovic,â the lead prosecutor, Dermot Groome, said on Tuesday in his opening statement.

The prosecution is expected to call 90 witnesses, the first of whom began to testify Tuesday. The witness appeared anonymously, his voice scrambled because he said he feared for his safety.